The influence of in vivo ageing on the surface properties of normal human erythrocytes will be studied in order to indicate which physicochemical changes during the red cell ageing process in vivo primarily control cell life span and senescent cell recognition by the reticuloendothelial system. Separation principles for the fractionation of red cells according to age and for the separation of reticulocytes from erythrocytes on the basis of age related membrane alterations will be screened by analytical techniques and then tested as liquid chromatography procedures. The behavior of aged Rhesus monkey red cells in these systems will be compared with that of human cells in order to establish a nonhuman primate model for in vivo studies of mechanisms of red cell ageing. The effectiveness of the fractionation techniques will be established by in vivo life span studies of radiolabelled fractionated Rhesus monkey red cells. Specific membrane modification techniques will be refined and employed to produce defined alterations in membrane properties which are indicated as critical to the in vivo recognition and elimination of senescent red cells. In vitro tests including erythrophagocytosis will be refined for monitoring membrane alterations as reflected by variation in the biological behavior of the cells and for examining the probable roles of environmental conditions and humoral immunological factors in the red cell recognition and destruction process in vivo. Selected structural and functional properties of the membranes of red cells aged in vitro and produced in hematological diseases where red cell life span is shortened will be compared with those of normal senescent cells in order to provide a better understanding of intrinsic cell properties and extrinsic conditions which result in shortened red cell survival in these circumstances.